The present invention relates to a summing network and particularly to dimensioning a summing network to optimize performance.
A summing network is used, e.g., in base stations in mobile systems for combining the base station""s transmitter branches to a common transmission antenna. In the following, the invention will be described by way of example by specifically referring to the summing network of a base station, although the invention is also applicable to other summing networks.
In a base station summing network, each transmitter branch comprises a transmitter and a narrowband band-pass filter, whose pass frequency corresponds to the transmission frequency used by the transmitter. Band-pass filters, i.e. combiner filters, prevent transmitters from interfering with each other""s functioning. In practice, each band-pass filter is usually tuned to the intermediate frequency of the corresponding transmitter in such a way that it passes the signal transmitted by the corresponding transmitter with a minimum loss to the summing network, and simultaneously prevents the signals of other transmitter at different frequencies from passing to the corresponding transmitter.
In order for a maximally large portion of the transmission power of the transmitters to be transferred to an antenna, the summing network has to be tuned with respect to the frequency channels used by the base station transmitters. The optimal electrical length of a summing network depends on the wavelength of the carrier of the signal to be transmitted. Strictly taken, a summing network is thus tuned only at one frequency. A summing network is generally tuned to the middle of the available frequency band, i.e. the M-frequency. In this case, the cables of the summing network with which the band-pass filters of the transmitter branches are coupled to the summing point are usually selected so that their length is xcex/2, wherein xcex is the wavelength at the M-frequency.
When moving away from the optimum frequency, i.e. usually the M-frequency, to the lower end of the available frequency band to the B-frequency or to the upper end to the T-frequency, the electrical lengths of the cables used no longer correspond to the value xcex/2, i.e. the electrical length of the cables is wrong. This causes load to the summing point, i.e. reactive mismatching. This load causes impaired reflection loss and pass loss, as well as narrowing of the bandwidths of the combiner filters.
In practice, the optimal frequency band of a summing network is too narrow to allow the frequency channels of the base station transmitters to be changed very much without having to deal with the tuning of the summing network. However, in practice, the frequency channels of base stations in mobile systems, for example, need to be changed between the B-frequency and the T-frequency. Automatically (by remote control) tuneable combiner filters becoming common, the need arises to facilitate the tuning of a summing network. A previously known solution, wherein an engineer visits the site of the base station and replaces the cabling of the summing network with cabling tuned to a new frequency band, is a too expensive and time consuming task.
Solutions are also previously known, wherein the summing network is provided with remotely controlled adjusting elements enabling the re-tuning of the summing network. However, these adjusting elements are relatively complex, and their manufacture and management increase costs.
The object of the present invention is to solve the above problem and to provide a solution that facilitates the tuning of a summing network. This object is achieved by the method of tuning a summing network according to the invention. The method of the invention is characterized by determining the load caused to a summing point by summing network branches, selecting a compensation element whose load effect corresponds substantially to the load caused to the summing point by the summing network branches, and coupling said compensation element to the summing network between the summing point and an antenna.
If the summing point is coupled via a conductor directly to the antenna, the compensation element can be coupled between the summing point and the antenna by coupling it to a conductor connecting them. However, if the summing point in a summing network is coupled to the antenna via another component (or other components), the compensation element is coupled between the summing point and the following component to a conductor connecting them. The term conductor refers to a transfer line for transferring signals between components. It may thus be e.g. a coaxial cable, a microstrip conductor or the like.
The invention also relates to a summing network comprising: a summing point having interfaces for coupling summing network branches to the summing point, and an interface for coupling the summing point to an antenna, and channel units arranged in the branches, and channel unit-specific band-pass filters, whose pass frequencies correspond to the frequency or frequencies of the corresponding channel unit. The summing network of the invention is characterized in that a compensation element is coupled to the summing network between the summing point and the antenna, the load effect of said compensation element corresponding substantially to the load caused to the summing point by the branches coupled to the summing point.
The invention is based on the idea of arranging a compensation element, whose load effect is substantially the same as the load caused to the summing point by the summing network branches, in the summing network between the summing point and the antenna. The use of such a compensation element eliminates the need to retune the summing network when the frequency channels of the channel units are changed. This is because the compensation element compensates for the mismatching caused by the electrical length of the summing network branches no longer being optimal in the new frequency band.
In this context, the concept channel unit refers to a transmitter, a receiver or a combination thereof. The solution of the invention is applicable for use both in a summing network in transmission use and in a summing network in reception use. A summing network in transmission use is used to sum the transmitter signals and apply them to a common transmission antenna. A summing network in reception use is used to branch signals received with the common antenna to the different receivers. In accordance with the invention, the summing point may be coupled either directly with a conductor to the antenna or, alternatively, via another component (or other components) to the antenna. An alternative is for the summing point to be coupled to the antenna via another summing point.
The most significant advantage of the method and summing network of the invention is thus that the cabling connecting the summing network branches to the summing point does not have to be replaced and the summing network does not need any other kind of tuning even if the frequencies of the channel units in the base station are changed.
In accordance with the invention, at its simplest, the compensation element could be a conductor whose length corresponds substantially to the total length of the conductors connecting the pass-band filters to the summing point. A significant additional advantage achieved by such a solution is a temperature-compensated summing network. This is because changes due to the change in the temperature of the conductor connecting the branches to the summing point and the conductor of the compensation element compensate for each other.
If the pass loss of the summing network is to be less than is achieved when a conductor is used as the compensation element, the compensation element can be a resonator, comprising e.g. a capacitor and a coil. In this case, the first poles of the capacitor and the coil are coupled to the connector between the summing point and the antenna, and the second poles are grounded.
The preferred embodiments of the invention are disclosed in the attached dependent claims 2 to 3 and 5 to 12.